1. Field of the Invention
The present invention relates generally to electrically switchable optical elements, such as application specific integrated elements including filters, lenses and switches, using wavelength locked feedback loops, and more particularly pertains to a combination of Electrically Switchable Bragg Grating (ESBG) technology with a wavelength locked feedback loop to provide variable focal length optical systems which automatically adjust the focal length of incident light.
2. Discussion of the Prior Art
Electrically Switchable Bragg Grating (ESBGxe2x80x94pronounced xe2x80x9cS-Bugxe2x80x9d) technology has recently become available from companies such as DigiLens Inc. These optical components possess electrically switchable diffractive optical elements or waveguides in a single solid-state device. The resulting device is capable of providing a unique blend of complex optical functionality within a tiny integrated package. The availability of this technology has opened up many new potential applications.
The ESBG technology has been presented in three basic variations of Application Specific Integrated (ASI) technology, namely Application Specific Integrated Filters (ASIF), Lenses (ASIL) and Switches (ASIS). The ASIL is discussed as a specific example below, with the understanding that other forms of ESBG technology can be substituted and used with dither wavelength locked feedback loops pursuant to the present invention.
An ASIL has different computer holograms imaged onto each layer. Each hologram corresponds to a different diffraction grating. For example, a diffractive lens with a variable focal length is capable of switching light at 35 xcexcsec, 10xc3x97 faster than electrical switching, and provides wavelength insensitive focusing in a compact package. A key element of this technology is a holographic, polymer-dispersed liquid crystal. While it contains materials common to liquid crystals used in flat-panel displays, the way in which the actual material builds the optical elements is different. A monomer and polymer liquid crystal are combined in such a way that upon exposure to a laser light fringe pattern, an area of pure polymer is created in the light fringes, and a mixture of monomer and polymer liquid crystal remains in the dark fringes. This plane of differing refractive indices is called a phase volume hologram. In the dark fringes, the liquid crystal is embedded in very small microdroplets.
When an AC voltage is applied across the plate, the microdroplets"" optical axes oscillate to match the refractive index of the monomer/liquid crystal area with that of the pure polymer. Thus the entire field looks like a clear window. With no voltage applied, the plane is a hologram containing a number of optical elements, essentially a diffractive lens. The result is the ability to switch the optical elements in and out of a xe2x80x9cdiffractive lens,xe2x80x9d independent of wavelength.
These hologram layers, typically 5 to 30 xcexcm thick, are deposited on a glass or plastic substrate. They can be stacked so that red, green and blue hologram optical elements can be contained in three separate layers with switching at 35 microseconds occurring within each layer. In this manner, the resultant optical switch can replace large refractive components, magnifying up to a factor of 20 to 40xc3x97 and providing full-color capabilities. Ultimately switching speeds may become as fast as 10 microseconds or less.
Instead of transmitting only a very narrow band of wavelengths, ASIL allows diffraction gratings that transmit red, blue or green light to be layered on top of each other. These layers can be switched on and off in turn at frequencies greater than 85 hertz, giving the full spectrum of color with no apparent flicker. Any optical effect that can be produced with conventional lenses can be written onto ASILs. ASIL devices are typically mounted on glass or plastic thin film substrates about 0.2 mms thick; the resulting devices are very lightweight and thus suitable for near-eye and handheld applications such as cell phone displays and mobile computing. As another example, combined with other technologies, ASILs can be used for high definition television (HDTV) projections.
There are many possible applications for this technology, including an electro/optical wavelength filter for wavelength multiplexing combined with a dynamic optical add/drop multiplexer. This area enables a new generation of optical switch components, including for example a multi-channel Dynamic Spectral Equalizer (DSE), which allows a real-time adjustment of power distribution within a wavelength multiplexing system. This ensures spectral flatness across all wavelength channels, which would otherwise be distorted by the highly non-uniform and dynamically varying gain profiles induced by cascaded erbium doped optical amplifiers (EDFA""s) and active Add/Drop functionality within the optical network. The resulting DSE is polarization-insensitive, eliminates the need for a multiplexing or demultiplexing layer within the network, and can potentially exhibit switching speeds as fast as 50 microseconds in either a free space or a guided wave optical design.
In telecommunications, all optical interconnects are currently converted to electricity where switching functions are performed and then converted back into light for transmission through the fiber. Optical switches can maintain transmission speed through high-bandwidth, fiber-based systems. With this technology, in addition to providing the ability to switch light and handle very complex routing systems, an asynchronous digital subscriber loop (ADSL) can switch between different frequencies of light. For example, in wavelength division multiplexing, the ADSL can selectively switch any particular frequency by effectively adding filters in each layer and switching among the layers.
For optical filtering, a selectable wavelength filter can be implemented for applications such as rear projection televisions and computer monitors. This has applications in compact disk and optical storage media, including volume holography. Optical designers have recognized the benefits of using holographic lenses in microdisplay applications because of their small size and lightweight. Holographic elements, however, only diffract a narrow bandwidth of light, typically 20 to 30 nm wide, thus limiting them to a single color, typically green. This is often seen in head-mounted displays for the military and in high-end industrial and medical applications. The ADSL technology is capable of switching the lens from clear to a red, blue or green holographic lens in stacked layers quickly enough to visually blend a flicker-free miniature display. The result is a full-color holographic display suitable for wearable computer displays and portable internet devices in general. This technology is used at the heart of several new devices that can xe2x80x9celectrically switchxe2x80x9d diffractive optical Bragg gratings on or off. This unique functionality opens up a broad range of components and subsystems used to control light, especially at the very high speeds required for optical telecommunications applications. This technology allows for recording of complex Bragg gratings which can encapsulate binary optical features, thereby reducing size, improving efficiency and lowering cost.
For wireless devices, an integrated sequential holographic lens is possible for enabling visual displays on compact devices such as cell phones. This has potential applications to handheld internet devices, CMOS device imaging, and wearable display technology. The long-term viability of portable and handheld devices, including smart phones, is dependent upon their providing an easy to use tool with which people can access information. Microdisplays (tiny high resolution displays on a chip) are part of the solution for these applications; ASIL can holographically encapsulate the prescriptions of multiple lenses within a slim glass (laminated) solid state device, saving space and decreasing weight, complexity and cost. The optical characteristics are calculated, computer-generated, and holographically recorded so that when the ASIL is embedded within a display, any single layer of the multi-layer
ASIL may be switched clear momentarily, allowing the next xe2x80x9cstackedxe2x80x9d layer""s optical properties to become activated. For instance, by switching a layer clear and synchronizing it with each of the microdisplay RGB sequential display cycles, the ASIL is able to solve the complex magnification problems associated with converting a microdisplay into a high visibility display.
The explanations herein discuss both wavelength and frequency, which have a reciprocal relationship (xcex=c/f, where c=speed of light), as is well known in the field of optics.
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Ideally, a WDM laser would produce a very narrow linewidth spectrum consisting of only a single wavelength, and an ideal filter would have a square bandpass characteristic of about 0.4 nm width, for example, in the frequency domain. In practice, however, every laser has a finite spectral width, which is a Gaussian spread about 1 to 3 nm wide, for example, and all real filters have a Gaussian bandpass function. It is therefore desirable to align the laser center wavelength with the center of the filter passband to facilitate the reduction of crosstalk between wavelengths, since the spacing between WDM wavelengths are so narrow. In commercial systems used today, however, it is very difficult to perform this alignmentxe2x80x94lasers and filters are made by different companies, and it is both difficult and expensive to craft precision tuned optical components. As a result, the systems in use today are far from optimal; optical losses in a WDM filter can be as high as 4 db due to misalignment with the laser center wavelength (the laser""s optical power is lost if it cannot pass through the filter). This has a serious impact on optical link budgets and supported distances, especially since many filters must be cascaded together in series (up to 8 filters in current designs, possibly more in the future). If every filter was operating at its worst case condition (worst loss), it would not be possible to build a practical system. Furthermore, the laser center wavelength drifts with voltage, temperature, and aging over their lifetime, and the filter characteristics may also change with temperature and age. The laser center wavelength and filter bandwidth may also be polarization dependent. This problem places a fundamental limit on the design of future WDM networking systems.
A second, related problem results from the fact that direct current modulation of data onto a semiconductor laser diode causes two effects, which may induce rapid shifts in the center wavelength of the laser immediately after the onset of the laser pulse. These are (1) frequency chirp and (2) relaxation oscillations. Both effects are more pronounced at higher laser output powers and drive voltages, or at higher modulation bit rates. Not only can these effects cause laser center wavelengths to change rapidly and unpredictably, they also cause a broadening of the laser linewidth, which can be a source of loss when interacting with optical filters or may cause optical crosstalk. Avoiding these two effects requires either non-standard, expensive lasers, external modulators (which are lossy and add cost), or driving the laser at less than its maximum power capacity (which reduces the link budget and distance). Lowering the data modulation rate may also help, but is often not an option in multi-gigabit laser links.
It would thus be highly desirable to provide a stable, optimal alignment between a laser center wavelength and the center of a Gaussian bandpass filter in order to optimize power transmission through such fiber optic systems and reduce optical crosstalk interference in optical networks.
Accordingly, it is a primary object of the present invention to provide electrically switchable optical elements using wavelength locked feedback loops.
A further object of the subject invention is the provision of a combination of ESBG technology with a wavelength locked loop to provide variable focal length optical systems which automatically adjust for the focal length of incident light.
The present invention concerns wavelength selective devices which encompass wavelength selective devices of all types including filters of all types including comb filters, etalon filters and rotatable disc filters and wavelength selective gratings of all types including Bragg gratings and array waveguide gratings.
It is an object of the present invention to provide a servo-control xe2x80x9cwavelength-locked loopxe2x80x9d circuit that enables real time mutual alignment of an electromagnetic signal having a peaked spectrum function including a center wavelength and a wavelength selective device implementing a peaked passband function including a center wavelength, in a system employing electromagnetic waves.
It is another object of the present invention to provide a servo-control system and methodology for WDM and DWDM systems and applications that is designed to optimize power through multi-gigabit laser/optic systems.
It is a further object of the present invention to provide a wavelength-locked loop for an optical system that enables real time alignment and tracking of any spectral device that selects a wavelength, such as a Bragg grating, in optical fibers and waveguides, etc., for use in WDM systems.
It is yet another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment of a laser with variable optical attenuators by offsetting an optical filter from a known transmission in optical fibers and waveguides, etc.
It is yet a further object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that may be used in light polarization applications.
It is still another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment and tracking of laser center wavelengths and filter passband center wavelengths in multi-gigabit laser/optical systems such that the optical loss of a WDM filter/laser combination is greatly reduced, thereby enabling significantly larger link budgets and longer supported distances.
It is yet still another object of the present invention to provide a servo/feedback loop for an optical system, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables real time alignment and tracking of laser center wavelengths and filter passband center wavelengths in multi-gigabit laser/optical systems such that lower cost lasers and filters may be used providing a significant cost reduction in the WDM equipment.
When employed in laser/optical networks, the system and method of the present invention may be used to tune laser diode devices, and/or compensate for any type of wavelength-selective element in the network, including wavelength selective filters, attenuators, and switches, in fiber Bragg gratings, ring resonators in optical amplifiers, external modulators such as acousto-optic tunable filters, or array waveguide gratings. This applies to many other optical components in the network as well (for example, optical amplifiers that may act as filters when operating in the nonlinear regime). Furthermore, the system and method of the invention may be used to implement less expensive devices for all of the above application areas.
Alternately, the system and method of the invention may be implemented to tune such devices for WDM and optical network applications, in real-time, during manufacture, e.g., tuning all lasers to a specific wavelength. This would significantly increase lot yields of laser devices which otherwise may be discarded as not meeting wavelength specifications as a result of manufacture process variations, for example.
The wavelength locked loop of the present invention enables a tighter control of wavelength, which allows an increased density of wavelength channels with less cross talk between channels in a wavelength multiplex system, which might typically include 32 or 64 channels or links. Pursuant to the present invention, each channel includes a separate wavelength locked loop which includes a separate source such as a laser and wavelength selective device such as a filter. Accordingly a wavelength multiplex system can include an array of 32 or 64 lasers and an array of 32 or 64 filters.